Difference between PLC and Relay Based Controller
In this article, we will discuss the main differences between relays and PLCs by considering various parameters, such as basic definition, components, function, memory, I/O, applications, etc. I will explain each point in detail, highlighting the differences between them, as well as the advantages of PLCs over relays and vice versa.
Table of Contents
- What is a Relay?
- What is a PLC?
- What is the Advantage of PLC over Relay?
- What is the Disadvantage of Relay?
- Comparison of PLC with Relay Logic Controller
- Difference between Relay and PLC
- Conclusion
What is a Relay?
An electrically operated switch which is used for switching a high-power circuit with the help of a low power circuit is known as relay. Hence, in simple words, a relay is nothing but an electrically operated switch.
A typical relay consists of a set of input terminals and a set of operating contact terminals. Where the input terminals are provided for inputting a single or multiple control signals.
Based on construction and design, there are two types of relays namely electromagnetic relays and solid-state relays. An electromagnetic relay consists of a coil wound around a soft iron core, where the iron core provides a path of low reluctance for the flow of magnetic flux. It also consists of a movable armature and a set of contacts. On the other hand, the solid-state relays do not consists of any moving part, they use semiconductor devices instead of electromagnets.
The relays are used in such applications where it is required to control the operation of an electrical or electronic circuit by a low power electric signal or where a number of circuits have to be controlled by one electric signal. Some practical applications of relays are in electronic circuits, home appliances, signaling and control in railways, motor control circuits, power generation and transmission systems, etc.
What is a PLC?
PLC stands for Programmable Logic Controller. A PLC is basically a microcontroller device that is used for automation of processes in industries.
A typical PLC consists of input module, processing element, program (PLC software), power supply and output module. A programmable logic controller (PLC) receives input signal from the input devices such as sensors and processes these input signals based on the instructions to produce the desired outputs.
A mechanism called HMI (Human Machine Interface) is used to interact with the PLC in real time. The HMI provides an interface on a digital screen through which a user can input the instructions in the PLC and can change the system settings.
A PLC is programmed by using different types of programming language such as block diagram, ladder logic, structural
What is the Advantage of PLC over Relay?
- If we compare the relay with PLC, PLC systems perform better in terms of work, accuracy. PLC requires less time, less wiring.
- PLC is easy to maintain. There will not be much physical work. PLC has more advanced functions as compared to manual work (hardware relay-base).
- These are the main reasons, why the industries are shifting to use PLCs over the relay.
What is the Disadvantage of Relay?
- The main disadvantage is the relay needs more wiring for performing different functions.
Comparison of PLC with Relay Logic Controller
- The computer-based PLC is most suitable compared to relay logic as input and output variables have only two states and hence are binary variables. This is most suited for PLC.
- Most of the control relays which are physically required in relay logic get replaced by software commands. This reduces the hardware.
- As the hardware is reduced, the problems related to hardware like wiring failure also get reduced considerably due to PLC.
- If there is any change in logic, it is not necessary in PLC to replace hardware or wiring. It is required to change the software which is very easy. Thus Programmed sequence can be easily altered in PLC.
- The special actions such as time delays, counters, registers can be easily achieved through the software in PLC.
- The programming cost in relay logic is high due to complicated wiring. But programming cost in PLC is low.
- The space required for PLC is less compared to relay logic controller.
- Due to modern technology, special semiconductor solid state devices are developed which can control high power actions with the help of low level commands from a computer.
- The relay logic controller is not reusable as PLC.
- The relay logic controller does not have arithmetic capability and memory as against this PLC has strong arithmetic capability and large memory space.
Difference between Relay and PLC
Both relay and PLC are the devices that are used for industrial automation, however there are several differences between them that are given in the following table −
